A tilt-rotor or tilt-wing aircraft typically employs a pair of rotor systems which are supported at the outermost end of a wing structure and are pivotable such that the rotors thereof may assume a vertical or horizontal orientation. In a horizontal orientation, the aircraft is capable of hovering flight, while in a vertical orientation, the aircraft is propelled in the same manner as conventional propeller-driven fixed-wing aircraft.
Currently, tilt-rotor/tilt-wing aircraft employ conventional fixed-diameter rotor systems which, in the aerodynamic and aeroelastic design thereof, attempt to blend the competing requirements of hovering and forward flight modes of operation. For example, with regard to hovering flight, it is generally advantageous to employ a large diameter rotor to improve hovering performance by lowering disk loading, reducing noise levels, and reducing downwash velocities. Conversely, a relatively small diameter rotor is desirable in forward flight to improve propulsive efficiency by minimizing blade aeroelastic properties, minimizing blade area, and reducing tip speed (Mach number).
Variable Diameter Rotor (VDR) systems are known to provide distinct advantages over conventional fixed-diameter rotors insofar as such systems are capable of adaptation to both modes of operation. That is, when the plane of the rotor is oriented horizontally, the rotor diameter is enlarged for improved hovering efficiency and, when oriented vertically, the rotor diameter is reduced for improved propulsive efficiency.
An example of a VDR system and VDR blade assembly therefor is shown in Fradenburgh 3,768,923 wherein each blade assembly includes an outer blade section which telescopes over an inner blade section, i.e., a torque tube member, so as increase or decrease the rotor diameter. The outer blade section includes a structural spar, i.e., the foremost structural element which carries the primary loads of the outer blade section, a leading edge sheath assembly and trailing edge pocket assembly, which sheath and pocket assemblies envelop the spar section to define the requisite aerodynamic blade contour. The torque tube member mounts to a rotor hub assembly and receives the spar member of the outer blade section. The torque tube member, furthermore, functions to transfer flapwise and edgewise bending loads to and from the rotor hub assembly while furthermore imparting pitch motion to the outer blade section.
The resultant torque tube/spar assembly forms an internal chamber for accepting a retraction/extension mechanism. The retraction/extension mechanism includes a threaded jackscrew which is disposed internally of the torque tube member and is supported at its inboard and outboard ends by means of journal bearing supports. Furthermore, the jackscrew may be driven in either direction by a bevel gear arrangement disposed internally of the rotor hub assembly. The jackscrew engages a plurality of stacked nuts which are rotationally fixed by the internal geometry of the torque tube member yet are permitted to translate axially along the jackscrew upon rotation thereof. Furthermore, centrifugal straps extend from each nut and are affixed, via a retention block, to the tip end of the spar member. As the jackscrew turns, the stacked nuts are caused to translate inwardly or outwardly, thereby effecting axial translation of the outer blade section. Systems relating to and/or further describing VDR systems/blade assemblies are discussed in U.S. Pat. Nos. 3,884,594, 4,074,952, 4,007,997, 5,253,979, and 5,299,912.
As with all rotor blade assemblies, the VDR blade assembly is exposed to various aerodynamic loads which result in multidirectional displacement of the blade assembly, e.g., flapwise and edgewise bending motions. Consequently, the torque tube and spar members, are appropriately designed and constructed of materials, e.g., advanced composites, which permit a degree a compliance to accommodate such motions. While the torque tube and spar members are readily fabricated from such materials, the complex geometry and loading conditions imposed on the jackscrew assembly require the use of high strength metallic materials which are less compliant and more susceptible to fatigue damage. Areas which are particularly prone to fatigue damage are those nearest the journal bearing supports. As a result, the service life of the jackscrew may be substantially lower than other components of the s blade assembly, e.g., the enveloping torque tube.
In addition to the structural requirements for accommodating blade excursions, other requirements associated with ballistic survivability must be satisfied to meet certain military requirements. For example, ballistic survivability requires that redundant load paths be established to maintain the structural integrity of the blade assembly in the event of a ballistic impact. Generally, multi-fiber materials such as fiber-reinforced composites offer superior ballistic properties due to the inherent redundancy provided by the fiber matrix and resistance to crack propagation. Metallic materials are characterized by high crack propagation rates and, consequently, are less desirable from a survivability standpoint. Insofar as the jackscrew is necessarily constructed of such metallic materials, it represents a point of vulnerability for the VDR blade assembly. This vulnerability is particularly evident when considering that the jackscrew assembly is the sole structural element which retains the outboard blade section.
Other retraction/extension mechanisms for VDR systems employ reeling assemblies wherein a spool member winds or unwinds a compliant cable which is affixed to the outboard blade section. More specifically, a main rotor shaft drives the spool member which, in turn, drives the rotor system about the rotor shaft axis. That is, the geometry of the shaft-driven spool member is such that the centrifugal force of the outboard blade section acting on the spool member creates a torque that balances the rotor shaft torque, thereby driving the rotor system. Such retraction/extension mechanisms are known in the art and are described in a November 1969 article published in Space/Aeronautics Magazine.
While retraction/extension mechanisms which employ reeling assemblies obviate the issues associated with fatigue, i.e., the cable is sufficiently compliant to accommodate blade motions, such reeling assemblies are not directly controlled by the pilot, but are indirectly controlled by the torque being applied to the rotor system. That is, the rotor blades are retracted and/or extended as a function of the collective pitch input being made to the rotor system. More specifically, as collective pitch is increased for cruise flight, the torque of the rotor shaft is increased to maintain constant rotor speed. At the same time, the increased collective pitch causes the rotor system, which is rotationally decoupled from the drive shaft, to decelerate with respect thereto. Consequently, the cables are caused to be wound about the spools, thereby retracting the rotor blade assemblies. As the rotor blades retract, the torque required to drive the rotor system decreases until such time that the rotor shaft torque matches that generated by the spool/cable arrangement. For rotor blade extension, the sequence is reversed. That is, as collective pitch is decreased, the torque required to drive the rotor system decreases and, consequently, the rotor system accelerates with respect to the rotor shaft. The cables, in turn, unwind from the spool thereby extending the rotor blade assemblies. As such, this system lacks the capability for positive control of rotor blade position, which control is particularly important in the event of a engine failure while the rotor blades are partially or fully-retracted. That is, a sudden decrease in rotor shaft torque results in rapid and uncontrolled blade extension. Should this event occur when the rotor system is in cruise flight, i.e., when the rotor systems are oriented vertically, such blade extension could result in catastrophic contact with the passenger cabin. Furthermore, the radial acceleration of the outboard blade sections would likely result in cable failure.
In addition to the undesirable features discussed above, the reeling assembly of the prior art imposes large torsional loads on the input drive shaft which drives the spool member. This will be appreciated by recognizing that the centrifugal loads acting on the outboard blade section (on the order of 40,000 lbs (178,000N)) must be reacted in torsion by the input drive shaft. Consequently, the diameter and/or the thickness of the input drive shaft must be appropriately configured to react the imposed loads. Such requirements adversely impact the design envelop and/or the weight of the VDR system.
A need therefore exists to provide a retraction/extension mechanism for a VDR system which is controlled by pilot input, provides improved fatigue life, and is ballistically survivable, without adversely impacting the design envelop and/or the weight thereof.